1. Field of the Invention;
This invention relates to a polymer Langmuir-Blodgett ultrathin membrane, hereafter abbreviated to LB membrane or LB ultrathin membrane. More particularly, it relates to a smooth and homogeneous LB membrane constituted of polyfumarate.
2. Related Art Statement;
As the methods for preparing a smooth and homogeneous organic ultrathin membrane having a uniform molecular orientation, it has been customary to use a Langmuir-Blodgett method, referred to herein as the LB method. The LB method means a vertical dipping method according to which a dilute solution of organic solvents immiscible with water is prepared and spread on a clean water surface, a gaseous membrane which is left after vaporization of the solvent is compressed in a horizontal direction to form a solid membrane with the molecules packed tightly together, and then a solid substrate plate is moved vertically with respect to the horizontal plane for transferring and stacking the solid membrane in plural layers on the surface of the solid substrate plate. The ultrathin film formed in this manner on the substrate is called the LB membrane, see for instance the literature by K. B. Blodgett, JACS., 55, 1007 (1935). On the other hand, a horizontal lifting method has also been evolved according to which the layers of the solid membrane is transferred by vertically moving the substrate plate so that the substrate plate is horizontally contacted with the surface of the solid membrane, see the literature by K. Fukuda, J. Colloid Interface, 54,430(1976). Currently, the ultrathin membrane formed on the substrate plate by the horizontal lifting method is also called the LB membrane. It is a feature of both the vertical dipping and horizontal lifting methods that a smooth and homogeneous membrane with a desired thickness and a uniform molecular orientation may be produced and that the produced membrane may range from an ultrathin membrane of the thickness of the order of a molecular thickness, e.g. monomolecular layer to a multilayered membrane of desired thickness produced by repeatedly transferring the monomolecular layers.
Alternatively, electrical elements such as varistors, thyristors, diodes, photodiodes, light emitting diodes and transistors as well as LSIs composed of these electrical elements, may be basically classified into metal/insulator/metal (MIM), metal/insulator/semiconductor (MIS), metal/semiconductor or Schottky element (MS), semiconductor/semiconductor(SS; p,n-junction) and semiconductor/insulator/semiconductor (SIS). The MIM, MIS and SIS elements need be formed with insulator layers and are usually prepared by oxidizing the surface of an aluminum or beryllium substrate or a silicon substrate to form an insulating film of reduced thickness of SiO.sub.2 or metal oxide followed by formation of a counter electrode. However, this technology cannot be adapted to metal or semiconductor substrates other than the abovementioned substrates. The adaptation to versatile MIS type elements such as diodes, photodiodes, light emitting diodes or field effect transistors is not possible when using inter alia semiconductors other than Si, including compound semiconductors. Therefore, all possible combinations can be achieved by using an organic insulating ultrathin membrane as the insulating layer. It is required that such insulating ultrathin membrane be of a thickness of not higher than 50 .ANG. and preferably not higher than 20 .ANG. while being smooth and homogeneous. Accordingly, it has been tried to apply the above described LB membrane as the electrical element.
As a typical example of such trials, formation of the LB membrane of straight-chain fatty acids having not less than 16 carbon atoms, or alkaline earth metal or cadmium salts thereof, has been considered extensively, see for example G. G. Roberts: IEE, Proc. Solid State Electron Device, Vol.2, p169, 1978. However, the LB membrane of these fatty acids or metal salts thereof are low in mechanical strength and heat resistance and therefore cannot be used practically. Accordingly, it has been suggested to form the polymerizable fatty acid into an LB membrane prior to polymerization followed by polymerizing the membrane or to polymerize on the water surface followed by forming an LB membrane. With the former method, however, the membrane is frequently constricted or cracked during polymerization. With the latter method, difficulties are encounted in setting the polymerization conditions and, above all, in transferring the membrane onto the substrate surface by the vertical dipping or horizontal lifting methods. Thus, it has been desired to produce a polymer LB membrane superior in mechanical strength and heat resistance.
In general, a soft linear chain high molecular material forms more or less intricately entangled strands in any dilute solution and is not suitable for being formed into LB membranes since gaseous membranes are not formed when spreading out the solution on the water surface. As an exception, an LB membrane of polypeptides and (.alpha.-olefin)-maleic anhydride alternating copolymers have been reported, see the literature by J. H. McAlear, VLSI Tec., Digest of Tec. Papers 82(1981); C. S. Winter et al, IEE Proc., Part I, Solid State Electron Devices, 130, 256(1983); and R. H. Tredgold et al, Thin Solid Films, 99, 81(1983). However, the former material is soluble only in a special multi-component solvent such as chloroform/trichloroacetic acid/methanol while trichloro acetic acid used as the essential component for maintaining the solubility is highly likely to deteriorate the metal surface used a the substrate. The latter material corrodes metal or semiconductor surfaces.